The ensure in APlayerState::OnRep_bIsInactive may be hit due to the player state being replicated before the game state. The check is necessary for making sure the game state is available, but the ensure is not needed since this is an expected possibility.
Immediately change the bIsInactive flag on a player state to something other than the default when a multiplayer game starts.
Depending on replication order or network conditions, the OnRep for bIsInactive may be called on a client before the game state has been created on that client.
Expected: The player state's OnRep would skip updating the game state's player array, and the game state would create a correct player array in PostInitializeComponents after its created on the client.
Actual: The ensure is hit.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-105202 in the post.